This invention is generally directed to improvements in acoustic surface wave devices. It is particularly directed to an acoustic surface wave filter for multiplexing a pair of signal inputs to develop a filtered signal output.
Multiplexing filters are typically used in applications where either one of two input signals are to be received, filtered, and output to further signal processing circuitry. For example, FIG. 1 shows a multiplexing filter 10 which is used with television games. It includes a pair of input ports for receiving either a channel 3 television signal or a channel 4 television signal, an internal channel 3 filter, an internal channel 4 filter, and an output port. Each internal filter is adapted to reject spurious signals which lie outside its passband so that, when either of the input signals are received, it arrives free of spurious signals at the output port.
The roles of the input and output ports may also be reversed. For example, the channel 3 and channel 4 signals may be applied to the output port for separation by the filter 10 so that each signal input appears at one of the designated input ports.
The advantages of constructing the filter 10 in the form of an acoustic surface wave device are well known and need not be discussed here. However, conventional acoustic filters do suffer from one or more of the practical drawbacks mentioned below.
Referring to FIG. 2, a conventional acoustic filter 12 is shown for implementing the functions of the general filter shown in FIG. 1. This filter includes an input port A coupled to an apodized channel A transducer 14, i.e., a transducer having overlapping regions of transducer fingers which are length-weighted along the transducer axis. An apodized channel B transducer 16 is coupled to input port B, and a uniform transducer 18 receives surface waves launched by the transducers 14 and 16 for generating a filtered, electrical signal at the output port. All these components are typically constructed on a substrate 20 of piezoelectric material. The frequency response of each channel in the filter 12 corresponds to the product of the frequency response of its associated apodized transducer and the frequency response of the uniform transducer 18, the latter device being used as the output transducer for both filter channels.
The problems associated with the filter 12 include undesirably high insertion losses due to the same uniform transducer 18 being common to both channels. For the same reason and because of bulk wave phenomena, the combined selectivity of the transducer 14 and transducer 18, and the combined selectivity of transducers 16 and 18, are degraded at frequencies which may overlap the two channels or occur at their boundaries. Out-of-hand spurious responses appear at approximately odd harmonics of the fundamental passband of each of the two channels.
Another conventional surface wave filter 22 is shown in FIG. 3. The substrate 24 of this filter carries a uniform transducer 26 coupled to an input port A and another uniform transducer 28 coupled to an input port B. Multistrip couplers 30 and 32 direct the acoustic energy associated with their received surface waves to an apodized channel A transducer 34 and to an apodized channel B transducer 36. The outputs of transducers 34 and 36 are coupled together as shown at an output port.
Although the filter 22 overcomes some of the problems associated with the filter shown in FIG. 2, its physical size is undesirably large and much of the substrate is unused. Its size could be reduced by modifying the geometry of the substrate if a way could be found to render harmless resultant acoustic reflections from the edges of the substrate.
These and other shortcomings of conventional acoustic surface wave devices render them less than perfectly satisfactory for use as multiplexing filters.